Fix a document memory leak in drag-and-drop.

Source/core/clipboard/Clipboard.h

 /*
  * Copyright (C) 2001 Peter Kelly (pmk@post.com)
  * Copyright (C) 2001 Tobias Anton (anton@stud.fbi.fh-darmstadt.de)
  * Copyright (C) 2006 Samuel Weinig (sam.weinig@gmail.com)
  * Copyright (C) 2003, 2004, 2005, 2006, 2008 Apple Inc. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
@@ skipping 61 matching lines @@
     void clearData(const String& type = String());
     String getData(const String& type) const;
     bool setData(const String& type, const String& data);
 
     // extensions beyond IE's API
     Vector<String> types() const;
     PassRefPtrWillBeRawPtr<FileList> files() const;
 
     IntPoint dragLocation() const { return m_dragLoc; }
     void setDragImage(Element*, int x, int y, ExceptionState&);
+    void clearDragImage();
     ImageResource* dragImageResource() const { return m_dragImage.get(); }
     void setDragImageResource(ImageResource*, const IntPoint&);
     Node* dragImageElement() const { return m_dragImageElement.get(); }
     void setDragImageElement(Node*, const IntPoint&);
 
     PassOwnPtr<DragImage> createDragImage(IntPoint& dragLocation, LocalFrame*) const;
     void declareAndWriteDragImage(Element*, const KURL&, const String& title);
     void writeURL(const KURL&, const String&);
     void writeRange(Range*, LocalFrame*);
     void writePlainText(const String&);
@@ skipping 42 matching lines @@
     ResourcePtr<ImageResource> m_dragImage;
     RefPtr<Node> m_dragImageElement;
 };
 
 DragOperation convertDropZoneOperationToDragOperation(const String& dragOperation);
 String convertDragOperationToDropZoneOperation(DragOperation);
 
 } // namespace WebCore
 
 #endif // Clipboard_h